1. Field of the Invention
The present invention relates to an image forming device such as a color laser printer.
2. Description of the Related Art
One type of conventional color laser printer includes a plurality of photosensitive drums. A different color toner image is formed on each of the photosensitive drums. The different-colored toner images are then transferred from the photosensitive drums one on top of the other onto an intermediate transfer member. The superimposed visible images are then transferred together from the intermediate transfer member onto a sheet in order to farm a color image on the sheet. Normally, a fixing unit is provided for fixing the color image onto the sheet.
Normally, the fixing unit includes a thermal roller and a pressing roller. When a sheet formed with a color toner image passes between the thermal roller and the pressing roller, the color toner image is thermally fixed onto the sheet.
Toner can undesirably cling to the thermal roller because of image forces exerted by the charge of the toner or Coulomb forces that accumulate at the surface of the thermal roller. This clinging toner can appear as ghost images in the visible color image fixed on the sheet. This problem can be referred to as electrostatic offset. Electrostatic offset can be prevented by adjusting the configuration of the laser printer in accordance with the charge-to-mass ratio and volume resistance of the toner. For example, a high-resistance element connected between the thermal roller and ground can be selected, or the surface of the thermal roller can be processed, in accordance with the charge-to-mass ratio and volume resistance of the toner.
However, color images are normally formed from a combination of yellow, magenta, cyan, and black colored toners. Normally, black toner includes a conductive coloring agent, such as carbon black and so has a lower electrical resistance than do the yellow, magenta, and cyan toners. When the different toners that form a color image have different resistances in this way, it is difficult to make adjustments in accordance with charge-to-mass ratio and volume resistance of the color toners. Therefore, it is difficult to effectively prevent electrostatic offset.
It is an objective of the present invention to overcome the above-described problems and to provide an image forming device capable of properly fixing color images formed from a plurality of developing agents with different electrical resistances, without the problem of electrostatic offset occurring.
In order to achieve the above-described objectives, an image forming device according to the present invention includes an image forming unit and a thermal fixing unit. The image forming unit uses a plurality of different color developing agents to form a color image on a recording medium. The plurality of different color developing agents are configured from binding resins with the same thermal properties. The plurality of different color developing agents include at least a high-resistance developing agent and a low-resistance developing agent. The high-resistance developing agent has a higher electrical resistance than the low-resistance developing agent and a lower charge-to-mass ratio than the low-resistance developing agent. The thermal fixing unit thermally fixes the color image onto the recording medium.
Because the high-resistance developing agent has a lower charge-to-mass ratio than the low-resistance developing agent, image forces exerted by the charge of the high-resistance developing agent are reduced, so the developing agent does not cling as easily to the thermal roller. Therefore, even a color image formed from a plurality of developing agent types with different resistance values can be properly fixed to the sheet without fear of electrostatic offset occurring.
An image forming device according to another aspect of the present invention includes an image forming unit that uses high-resistance developing agent and the low-resistance developing agent that both have a charge-to-mass ratio of 20 xcexcC/g or greater. However, the high-resistance developing agent is configured from a binding resin with a glass transition point that is lower than a glass transition point of a binding resin that configures the low-resistance developing agent.
Because the charge-to-mass ratio of all developing agents is adjusted to 20 xcexcC/g or greater, the developing agents have sufficient clinging force so that disturbance in color images caused by generation of steam from the sheet can be effectively prevented. Because the high-resistance developing agent is configured from a binding resin that has a glass transition point lower than the glass transition point of the binding resin that configures the low-resistance developing agent, the high-resistance developing agent melts more easily so that the high-resistance developing agent can be fixed to the recording medium using the same fixing temperature as for the low-resistance developing agent, and consequently the high- and low resistance developing agents can be fixed simultaneously.
An image forming device according to another aspect of the present invention includes an image forming unit that uses different color developing agents that each have different electrical resistances and a volume resistance of 1010.5 ohm cm or greater. The thermal fixing unit includes a heating member and a pressing member. The heating member contacts a surface of the recording medium formed with the color image. The pressing member presses the heating member through pressing contact with an opposite surface of the recording medium. At least one of the heating member and the pressing member has a surface resistance from 106 to 1010 ohm and the other has a surface formed from an electrically insulating material.
Electrostatic offset can easily occur when the volume resistance of the developing agents is 1010.5 ohmxc2x7cm or greater. Furthermore, developing agent types with different resistance values also exert different image force by the charge of the developing agent. Therefore, it is difficult to fix all the developing agent types simultaneously under the same conditions.
The member formed with a surface made from an electrically insulating material can develop a negative charge as the recording medium rubs against it during fixing operations. However, because the other member is formed with a surface resistance of 106 to 1010 ohm, a portion of the negative charge can escape through the surface of the other member. For this reason, charge of the developing agent will not be excessively influenced during fixing operations by the negative charge generated on the surface of the member so that proper fixing operations can be performed and color images can be properly fixed without generation of electrostatic offset.